This invention relates to capacitive transducers and, more particularly, to capacitive transducers with a feedback control.
Capacitive transducers are widely used to measure displacement or rotation of an element, such as the linear position of a machine tool or the angular position of a mirror or of an optical grating. They offer many advantages as they can be made to resolve extremely small motions, such as the tilt of a coastline with tidal changes, they are relatively inexpensive to build, they can report values of position nearly instantaneously, and they can be made using lightweight parts since it is important not to significantly increase the inertia of the instrumented element.
As described in many publications, capacitive transducers for sensing rotary motion include at least two fixed capacitance plates and a moving member mounted on a centrally located rotating shaft. The transducers can be divided into two types. The first type has a dielectric plate as the moving member, and the second has a conducting plate as the moving member. The moving dielectric member is shaped to alter the dielectric constant and, in turn, the capacitance of the transducer depending on its rotary position. The moving conducting plate is kept at the same potential as one of the electrodes and is shaped to prevent energy transfer from one electrode to the other depending on its instantaneous position.
The first type of capacitive transducers is disclosed in U.S. Pat. No. 3,517,282 to Arthur Miller. The transducer includes two stationary capacitor plates spaced apart and a dielectric plate mounted in between on a centrally located rotatable shaft. The capacitance between the stationary plates depends on the position of the rotating dielectric plate. The dielectric plate may have regions coated with a metallic material.
In U.S. Pat. No. 4,864,295, Rohr also describes the moving dielectric capacitance sensing system. The capacitance system is formed by two cooperating fixed capacitive members; the first one is made of a ring shaped plate enclosing a rotating shaft, and the second is made of four arcuate segments symmetrically positioned with respect to the shaft. Located in between the two fixed members is the moving dielectric member mounted on the shaft. The dielectric member has a butterfly shape and changes the capacitance depending on the rotary position of the shaft. The system also includes an additional capacitor, Cr, not affected by the moving dielectric but to some extent by the field of the two capacitive members, which is described to detect capacitance changes due to temperature variations. The output from the Cr capacitor is connected in a AGC feedback arrangement to the oscillator that drives the capacitance sensing system.
The second type of capacitive transducers is described by Parnell in U.S. Pat. No. 3,668,672. The transducer includes three conductive plates arranged in parallel, wherein two outer plates are stationary and the inner plate is mounted on a rotatable shaft. An electrical source signal is applied to one of the outer plates while the other outer plate, called receptor plate, and the inner plate, called screen plate, are maintained at substantially the same potential. The capacitance of the transducer, measured at the receptor plate, varies depending on the position of the screen plate that corresponds to the position of the shaft.
In U.S. Pat. No. 3,732,553, Hardway, Jr. discloses an improvement of the capacitive transducer of Parnell. The source plate of the transducer is divided into 2.sup.n sectors and each sector is connected to one of two sources of input electrical signals of opposite phase. The shield plate includes 2.sup.n-1 shielding lobes and the receptor plate includes 2.sup.n- active areas connected to the amplifier. The shield plate is maintained at ground or at some low signal value with respect to the input signals, and the amplifier includes a negative feedback circuit which clamps the signal level on the active areas to some low signal level with respect to the input signals. This arrangement is described to minimize the effect of stray capacitances and fringing between the capacitances. The capacitances are aligned with respect to each other so that a null or no signal position is obtained when the capacitance between the active areas and the sectors of the source plate connected to input signals of one phase, equals the capacitance between these active areas and the other sectors of the source plate connected to input signals of opposite phase. Movement of the shield plate in one specified direction from the null position upsets this balance in the opposite direction from the null position and results in a negative output signal. The amplitude of the output signals is proportional to the distance moved in the respective direction.
Frequently, a capacitive transducer is used in conjunction with a motion actuator that dissipates heat. The heat causes errors between the reported position values obtained during their initial calibration and the position values actually obtained in operation. This is the case, for example, in optical scanners, wherein a limited rotation motor drives a shaft with a mirror mounted thereon. For many reasons, such as preserving the dynamic bandwidth of the device, it is of interest to keep the capacitative sensor tightly coupled mechanically to the drive motor. Such tight coupling creates a path for heat conduction and exposes the transducer to the heat dissipation of the motor. While it is possible to measure the errors introduced by such heat transfer and to develop appropriate corrections for the transducer, it is costly and impractical to do so. Similar errors are induced by ambient temperature level changes and by the aging of components.
The above-described errors may be eliminated by calibrating the output of the transducer at sufficiently frequent intervals. The calibration is done at known angular positions by causing the scanner to address these positions. The output values are then compared to those obtained during the original calibration to generate appropriate corrections in the system driving the scanner. This technique requires that the useful scan function be temporarily interrupted more or less frequently to perform the calibration, and it also adds system complexity by the addition of optical components and the routines required for calibration and scan correction.
There is still a need for a capacitive transducer that is stable with respect to the commonly observed tendency for the output of all capacitive transducers to drift when their structures or the electrical circuits connected thereto age or are exposed to heat sources or to changes of ambient temperature and humidity.